1. Field of the Invention
The present invention is directed to a system for the identification of pneumatic tires which have an electronic tag embedded in their thickness. More particularly, it relates to the structure of antennas adapted for communication in such systems.
2. Discussion of the Background
It has been sought for a very long time to incorporate, in a pneumatic tire, transponders making identification possible by electromagnetic signals, notably to be able to control the flow during the production of pneumatic tires. One of the problems encountered in incorporating transponders in a pneumatic tire is that the orientation of the transponder with respect to the interrogation station, and more particularly with respect to the antenna used for the interrogation, generally is not known. Now, unless the orientation of the antenna corresponds to the orientation of the antenna of the transponder, the read or write distance is very severely reduced. If the antenna and transponder are perpendicular with respect to one another, then communication is not even possible.
The available solutions in the prior art for solving this problem are essentially of three different types. First of all, it has been proposed to scan the pneumatic tire containing the transponder with an inspection antenna to try all possibilities of relative position and orientation between the inspection antenna and the antenna of the transponder incorporated in the pneumatic tire until a signal is detected. By way of example, U.S. Pat. No. 3,160,865 proposes a communication device whose antenna rotates.
It can also be conceived to visually locate, on the surface of the pneumatic tire, the position and orientation of the transponder it contains. This introduces an additional constraint during the production of the pneumatic tire, and this constraint is generally unacceptable.
Finally, it can be conceived to solve this problem by multiplying the interrogation antennas so as to arrange them according to all possible orientations so that at least one of them is able to communicate with the antenna of the transponder. This solution not only multiplies the cost of the interrogation device, but further involves specific technical difficulties that will be explained below.
In FIGS. 1 and 2, the problem posed by communication with a transponder incorporated in a pneumatic tire is illustrated. In FIG. 1, a pneumatic tire 1 comprises a transponder with its antenna 2 implanted in the sidewall or in the bead thereof. Generally, the transponder 2 is implanted in the upper part of the bead, more specifically in the bead filler. Pneumatic tire 1, as it appears in FIG. 1, is placed flat on a conveyor belt and travels in direction P toward a communication antenna 3. The antenna 3 is placed flat under the conveyor belt or on it. Basically, antenna 3 forms a plane parallel to the pneumatic tire 1. This antenna 3 consists of numerous turns 30 wound on a support made of nonmagnetic material, each turn 30 comprising the rectangle that appears in FIGS. 1 and 2. If current is fed to such a winding, a magnetic flux .phi.3 is generated whose orientation is perpendicular to the plane of FIG. 1, and whose direction depends on the direction of the applied current. The magnetic axis of such a winding is defined as being the orientation of the resulting flux (south-north) that such a winding would develop if it were fed direct current.
Transponder with its antenna 2 is implanted in the pneumatic tire at a location that is unknown. All the possible positions are symbolized in FIG. 1 by positions a1, b1, c1, . . . , h1. The transponder comprises a communication antenna 2 made by winding a large number of turns on a small, elongated, cylindrical ferrite core and which can be schematically viewed on the drawings. The magnetic axis (according to the conventional definition given above) of such an antenna 2 is formed in the plane of FIG. 1, along the major axis of the oval diagramming the position of the transponder in FIG. 1. If, in the pneumatic tire, the azimuth position of the transponder (hence of its antenna 2) is unknown, it is known, on the other hand, that it is located at the level of the bead filler, with its antenna 2 oriented as indicated above. In other words, only a single degree of freedom to characterize the implantation of the antenna 2 of the transponder remains unknown.
If the transponder occupies position b1 or f1, then the coupling between the antenna 2 and antenna 3 is never possible regardless of the progress of the pneumatic tire toward antenna 3. The double arrows seen on antenna 3 in FIG. 1 represent the flux surrounding the turns of the winding in the immediate vicinity of these turns 30. This is the orientation that would be assumed by the needle of a compass placed a few centimeters above antenna 3, i.e., about at the level at which the sidewall of the pneumatic tire travels. It is understood that, in this embodiment of the prior art, the lines of flux will always be perpendicular to the magnetic axis of the antenna 2 of the transponder if the latter occupies position b1 or f1, and thus coupling is not possible with antenna 3.
If, on the other hand, the transponder occupies position a1, then when the pneumatic tire is just above antenna 3, coupling is possible with position a1 indicated on antenna 3. The same reasoning leads to the observation that coupling will be possible if the transponder occupies positions c1, e1 or g1.
If the transponder occupies positions h1 or d1, then, with antenna 3 as it appears in FIG. 1, coupling is random or more difficult because the flux emanating from the antenna of the transponder is never aligned with the direction of maximum sensitivity of antenna 3, as is shown by the double arrows on antenna 3 in FIG. 1. On the other hand, if the same antenna 3 occupied the entire width of the belt, and more specifically if its width were at least equal to the diameter of the implantation of the transponder 2 in the pneumatic tire 1, then communication would be possible since the flux surrounding transverse turns 30 of antenna 3 would be aligned with the magnetic axis of the antenna 2 of the transponder.
FIG. 2 is a perspective representation of the same antenna 3 in which magnetic axis .phi.3 has been represented in the form of an arrow with a solid line, and the lines of flux surrounding the turns have been represented in the form of broken lines. Also shown in FIG. 2 is the arrangement of each turn 30 with respect to the rectangular frame supporting them.
In summary, it is seen that the reading or detecting of the transponder will always be possible if the transponder occupies positions near positions a1, c1, g1 or e1 (50% probability), that it will always be impossible if the transponder occupies positions such as b1 and f1 (25% probability) and that the reading can be random for positions h1 and d1 (25% probability), as a function of the exact position of the pneumatic tire 1 on the conveyor belt and of the position and the size of the antenna 3 with respect to this conveyor belt.